1. Field of the Invention
The invention relates to dielectric ceramic compositions, and more particularly high frequency dielectric ceramic compositions, which have a dielectric constant of at least 45, a Q value of at least 43,000, and a stable resonant frequency temperature coefficient, thereby being applicable to electrical parts for satellite or mobile communication appliances.
2. Description of the Related Art
In order to be used as parts for satellite or mobile communication appliances, a high frequency dielectric desirably has a high Q value, a stable resonant frequency temperature coefficient(xcfx84f), and a high dielectric constant (xcex5r). In general, high frequency dielectrics are divided into three classes: high dielectrics having a dielectric constant of at least 60, mid dielectrics having a dielectric constant between 40 and 60, and low dielectric loss materials having a dielectric constant of 30 or less.
The dielectrics having the high dielectric constant typically include BaOxe2x80x94Re2O3xe2x80x94TiO2(Re=rare earth) type and (Pb,Ca)ZrO3 type. The dielectrics having the low dielectric loss include, for example, Ba(Zn1/3Ta2/3)O3 and Ba(Mg1/3Ta2/3)O3.
A dielectric having the mid dielectric constant (xcex5rxe2x89xa745) and a high Qxf ( greater than 43,000) has not been developed. Thus there is a demand for such dielectric substance.
Meanwhile, known existing dielectrics having a mid dielectric constant include (Zr,Sn)TiO4, (1xe2x88x92x)LaAlO3-x(Ca(or Sr) TiO3, (1xe2x88x92x)La(Zn1/2Ti1/2)O3-xCa (or Sr)TiO3, (1xe2x88x92x) NdAlO3-xCaTiO3, etc.
The (Zr,Sn)TiO4 has a Qxf of 45,000, an xcex5r of 38 and a temperature coefficient of near 0. This material has a high Q value, however, has a disadvantage in having a low dielectric constant.
The dielectrics having a compositional formula represented by (1xe2x88x92x)LaAlO3-x(Ca(or Sr)TiO3 show various dielectric properties depending on their composition ratios. In particular, it has been reported that a dielectric material having a composition of 0.35LaAlO3-0.65 (Ca(or Sr)TiO3S shows microwave dielectric properties with a Qxf of 47,000, xcex5r of 38 and xcfx84f of 5 ppm/xc2x0 C. (See U.S. Pat. No. 5,356,844). However, this ceramic material also has a low dielectric constant, although it has a relatively high Q value. Accordingly, there are problems to be applied in practical elements.
Similarly, the ceramics having a compositional formula represented by (1xe2x88x92x) La (Zn1/2Ti1/2)O3-xCa (or Sr) TiO3 show various dielectric properties depending on their composition ratios. In particular, a dielectric composition of 0.5La(Zn1/2Ti1/2)O3-0.5Ca(or Sr)TiO3 shows good properties with a Qxf of 50,000, xcex5r of 38 and xcfx84f of 5 ppm/xc2x0 C. However, these ceramic compositions also have a low dielectric constant and thus cannot be readily used in manufacturing practical elements.
In 1994, dielectric compositions having a formula represented by (1xe2x88x92x)NdAlO3-xCaTiO3, which show good dielectric properties with a Qxf of 47,000, xcex5r of 43 and xcfx84f of xe2x88x921 ppm/xc2x0 C. However, these dielectrics have a disadvantage in that it can be hardly sintered and thus has to be sintered at a temperature of 1450xc2x0 C. or higher (See U.S. Pat. No. 6,025,291).
Therefore, there are demands for a dielectric having excellent sintering properties as well as a mid dielectric constant (xcex5r greater than 45), a high Q value (Qxf of 43,000 or higher) and a good resonant frequency temperature coefficient.
Therefore, the present invention has been made in order to solve the above-described problems involved in the dielectric ceramic compositions of (1xe2x88x92x)NdAlO3-xCaTiO3. Now, we have completed the present invention after conducting repeated researches in order to lower the sintering temperature while maintaining at least dielectric properties of the dielectric composition of (1xe2x88x92x)NdAlO3-xCaTiO3.
An object of the present invention is to provide a new high frequency dielectric ceramic composition showing excellent sintering properties as well as good dielectric properties with a dielectric constant of 45 or more, a Q value of 45,000 or higher and a resonant frequency temperature coefficient of near 0.
In order to achieve the above object, the present invention provides dielectric ceramic compositions having a formula represented by (1xe2x88x92x)Nd(Ga1xe2x88x92yAly)O3-xCaTiO3, wherein x refers a mole fraction of CaTiO3 to the composition and satisfies an expression of 0.5xe2x89xa6xxe2x89xa60.8, and y refers a mole fraction of Al to Ga and satisfies an expression of 0xe2x89xa6yxe2x89xa60.9.
In accordance with the present invention, Ga is used as a main component since it has a Q value equal to Al but can lower the sintering temperature by about 100xc2x0 C., thereby increasing a mass-productivity of a dielectric resonator. Also, by substituting Ga for a part of Al, it is possible to readily control the resonant frequency temperature coefficient, as there coexist three phases.
Meanwhile, the present invention sets limits to x, a mole fraction of CaTiO3, as 0.5xe2x89xa6xxe2x89xa60.8. When x is less than 0.5, the dielectric constant is decreased excessively and the resonant frequency temperature coefficient becomes a too great negative (xe2x88x92) value. When x exceeds 0.8, the Q value is decreased excessively and the resonant frequency temperature coefficient becomes a too great positive (+) value. Therefore, it is not preferable for the composition to have x out of the above-described range.
Also, the present invention suitably sets limits y, a mole fraction of Al as 0xe2x89xa6yxe2x89xa60.9, preferably 0xe2x89xa6yxe2x89xa60.5. When y exceeds 0.9, the added amount of Al became larger and it is thus substantially difficult to expect a lowering of the sintering temperature.